This invention relates to a process for the preparation of polyisocyanates by thermally splitting the corresponding carbamates. More specifically, it relates to a process in which the thermal splitting is carried out in the presence of an auxiliary selected from hydrogen chloride, inorganic and organic acid chlorides, compounds having an alkylation effect and organotin (IV) chlorides.
It is known that isocyanates may be made by the thermal splitting of carbamic acid esters (See, e.g., H. Schiff, Ber. der Dtsch. Chem. Ges., Vol. 3, p. 649 and A. W. Hofmann, Ber. der Dtsch. Chem. Ges., Vol. 3, p. 653). U.S. Pat. No. 2,409,712 describes a process in which the products formed by splitting carbamic acid esters are prevented from recombining either by rapid distillation of the product or by introduction of the product into a cyclohexane-water mixture. Although the process described is suitable for the preparation of isocyanates, it produces only moderate yields. Moreover, it is very expensive to practice on an industrial scale.
The processes disclosed in U.S. Pat. Nos. 3,962,302 and 3,919,278 are also carried out in a liquid phase (i.e., in the presence of inert solvents). In each of these processes, the thermal splitting of monofunctional and difunctional carbamic acid esters takes place in the absence of any type of additive in the inert, high boiling solvent. The products of the splitting reaction (i.e., isocyanate and alcohol) are distilled off continuously from the reaction medium and separately condensed. Such processes are disadvantageous in that only moderate volume-time yields can be obtained.
In order to increase the reaction rates, the use of basic catalysts has been proposed in German Auslegeschriften Nos. 1,016,699; 1,022,222 and 1,028,342. However, it is known (see, for example, J. Appl. Pol. Sci., Vol. 16, P. 1213) that the use of basic catalysts leads to increased formation of solid and insoluble by-products. Consequently, only moderate isocyanate yields are to be expected when a basic catalyst is used. Furthermore, the formation of solid by-products is extremely undesirable from a practical standpoint because sparingly soluble ureas and isocyanurates can lead to the blockage of pipes and other parts of the apparatus, with serious consequences. Moreover, carbon dioxide and gaseous olefins which are formed during the decarboxylation reaction of carbamic acid esters can cause pronounced gas charging of distillation columns.
The suppression of side reactions is particularly important during the preparation of nondistilling isocyanates (for example, polyisocyanate mixtures in the diphenyl methane series) because all non-volatile by-products will remain in the product isocyanate thereby reducing the quality of the isocyanate.
U.S. Pat. No. 3,919,279 and German Offenlegungschrift No. 2,635,490 describe processes in which the thermal splitting of carbamic acid esters is carried out in inert solvents and in the presence of metal catalysts. The use of these catalysts (primarily Lewis acids) generally results in yields which are higher than those achieved with basic catalysts. However, the problem of formation of undesirable by-products is not eliminated in either of these processes. Consequently, the above-mentioned processing disadvantages are also encountered when using these metal catalysts. These processes are subject to other serious disadvantages when non-distilling isocyanates are being produced because the catalysts used in the splitting operation remain in the splitting basin.
Japanese patent application No. 54-39002 (1979) discloses a process for the thermal splitting of TDI-bis-ethyl urethane which is carried out in an inert solvent in the presence of organic H-acidic compounds (for example .beta.-diketones) without using metal catalysts. The formation of undesirable by-products would be expected to be prevented or substantially reduced by such additions of H-acidic compounds. In this process, however, only relatively small volume/time yields are possible due to the lack of effective catalysts. The economic feasibility of this process is therefore questionable.